1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to deceleration control for elevator systems of the traction type.
2. Description of the Prior Art
An elevator car must land smoothly and accurately, regardless of load and travel direction. This accuracy and smoothness is obtained by continuously controlling the deceleration torque. For example, it is common to use a direct current drive motor for driving the traction sheave, either directly or through a reduction gear, along with variable direct current voltage control which controls the magnitude and the polarity of the direct current voltage applied to the drive motor in response to some type of feedback arrangement. Direct current systems provide the desired smoothness and accuracy, and are almost universally used in gearless elevator systems which operate at contract speeds of about 500 FPM and higher.
Geared traction elevator systems conventionally use variable voltage direct current drive motors and control at the upper end of the geared speed range, such as about 200 to 500 FPM, and alternating current drive motors with rheostatic, or other suitable control, at the lower end of the speed range such as about 50 to 200 FPM. Alternating current drive systems are less costly than direct current drive systems, but in general are not as smooth and accurate. The economic advantage of an alternating current drive, however, is very attractive and many different arrangements have been used for improving the landing smoothness and accuracy of such systems, such as (a) using large flywheels with single or two-speed alternating current drive motors, (b) open loop control of a variable force friction brake with a fixed braking distance, or open loop control of braking distance with fixed braking force, with adjustments according to load, speed, load and speed, load and direction, speed and direction, or (c) closed loop control of a friction brake responsive to the error between a feedback signal responsive to actual car speed and a predetermined braking pattern of desired car speed at that particular point in time.
The above-mentioned arrangements have achieved various degrees of success, with the closed loop velocity controlled braking being the most accurate of the prior art systems, under all conditions of load, travel direction and car speed. It would be desirable to provide a new and improved closed loop controlled braking arrangement for elevator systems which is highly accurate, such as landing accuracies of .+-.0.25 inch, and with repeatable highly accurate landings under all conditions of load, speed and travel direction. Further, the new and improved system should provide a short floor-to-floor time without jolts, or other speed discontinuities which might cause passenger discomfort, and the system must be implemented with a low cost control package which includes an alternating current drive motor and reduction gear.